1. Field of the Invention
The present invention relates generally to a traction control system for an automotive vehicle in which both deceleration slip braking system (so called, ABS control) and acceleration slip braking system (so called, TCS control) are mounted.
2. Description of the Background Art
A Japanese Patent Application First Publication No. Heisei 1-301420 published on Dec. 5, 1989 exemplifies a previously proposed traction control system in which both deceleration slip brake control system (anti-lock brake controlling (ABS control) means and acceleration slip brake controlling system (brake controlling means from among driving force controlling means) are mounted.
In the previously proposed traction control system, when a switching operation between the deceleration brake control system and acceleration slip brake control system in both systems of which a brake control actuator is commonly used is carried out with the same tire wheel speed information being served as a control information, an initial control condition after the switching operation is set according to a controlled state before the switching operation.
Then, in the previously proposed traction control system disclosed in the Japanese Patent Application Publication identified above, in a case where the request of operation for the acceleration slip brake control during the deceleration slip brake control is issued, the acceleration slip brake control is immediately switched to the deceleration slip brake control. In a similar case where the request of operation for the deceleration slip brake control during the acceleration slip brake control is issued, the deceleration slip brake control is immediately switched to the acceleration slip brake control.
In this way, due to the same switching control of the switching operation, a control hunting (interference) is generated due to repetition of both the deceleration slip control and acceleration slip control.
Particularly, a braking performance is reduced, mainly caused by the deceleration slip brake control.
For example, suppose a case where the braking operation is carried out during the run on a low friction coefficient road surface and the switching is made from the acceleration slip brake control to the deceleration slip brake control.
As shown in FIG. 9, when rotation speeds of non-driven tire wheels are reduced due to the control of deceleration slip, the traction control system erroneously determines that a difference in speed between the non-driven tire wheels and driven tire wheels is generated due to the acceleration slip so that the switching from the deceleration slip control to the acceleration slip control is immediately carried out. Thereafter, when the traction control system determines that the deceleration slip occurs during the acceleration slip brake control, both brake controls are sequentially repeated such as to be switched to the deceleration slip brake control.
In addition, for example, in a case where the request of operation to start the deceleration slip brake control is issued only for one of the right and left front tire wheels, the acceleration slip brake control is immediately halted and is switched to the deceleration slip brake control. At this time, the vehicular stability is conversely reduced. This result has a cause that why the operation request for the deceleration slip brake control for one of the left and right front tire wheels is issued is that the vehicle receives a disturbance on the road surface, receives a variation of tire wheel load, and/or runs on a road segment such that only one of the left and right front tire wheels runs on a low friction road.
Furthermore, in a case where the request of operation for the acceleration slip brake control is issued during the deceleration slip brake control, the previously proposed traction control system immediately switches the control to the acceleration slip brake control. Therefore, an incremental pressure from an accumulator is added to a remaining pressure in a wheel cylinder caused by the deceleration slip brake control so as to result in an excessive braking control and a generation of a deceleration feeling by a vehicular driver.
In addition, when the acceleration brake control is carried out after the switching of control, a brake remaining liquid during the deceleration slip brake control is left in a reservoir of the brake control actuator. Therefore, a sufficient pressure decrease control cannot be achieved, a tendency to result in a tire slippery rotation may occur, and consequently, a deceleration feeling by the vehicular driver may be brought out. The deceleration feeling is such that although the vehicular driver depresses an accelerator pedal to accelerate the vehicle, the vehicle does not give the acceleration feeling to the driver but, in turn, gives the driver to feel as if the vehicle were decelerated due to the slippery motion of the driven tire wheels toward which an engine torque is transmitted.